This invention relates to a method for determining the volume of a section of an underground cavity such as an underground storage cavern.
Liquefied petroleum gases such as butane or propane are sometimes stored in underground caverns located hundreds of feet below the ground. These caverns are most typically formed by first lowering a tube down a borehole and then passing a flow of water through the tube so as to exit the lower end of the tube and circulate through salt deposits. This dissolves the salt so as to gradually carve out the cavern. A large amount of brine (substantially saturated salt solution) results, several times the volume of the cavern, some of which fills the cavern and the borehole and the remainder of which is passed to a brine pond. A fluid desired to be stored in the cavern is simply passed into the borehole so as to form an interface between the fluid and the brine so that the interface moves progressively downward as fluid is injected. Brine is consequently urged upward through the above-mentioned tube, which remains in the cavern after the cavern's formation, and into a brine pond.
One serious problem in storing fluids such as liquefied hydrocarbons in a storage cavern is the potential for leakage of the stored fluid into the surrounding formation which can lead to contamination of water deposits from which drinking water is pumped. Consequently, the United States Environmental Protection Agency and various states have required that caverns for storing certain specified hydrocarbons be tested for "mechanical integrity". In other words, th caverns must be tested to determine the rate of leakage therefrom. Such testing usually involves injection of a test fluid such as nitrogen into a brine-full cavern so as to form an interface between the brine and the nitrogen which is positioned somewhat below the bottom of the borehole. The borehole is then appropriately capped and the interface observed over a period of time, typically about two days. Any rise in the interface indicates leakage of nitrogen into the surrounding formation, which in most instances occurs in the vicinity of the casing shoe at the bottom of the borehole or through the casing. Thus, when the interface rises from a lower level to an upper level where the levels define the upper and lower boundaries of a particular cavern section, the volume of this section must be known in order to determine the volume of test fluid which has leaked into the surrounding formation. Prior methods of determining the volume of cavern sections suffer from many disadvantages and give rise to many of the problems and inaccuracies associated with mechanical integrity testing of caverns.
According to one prior method, which is described in U.S. Pat. Nos. 4,455,869 of Broussard et al and 4,523,453 of Faul et al, liquid nitrogen from a tank is vaporized and injected into a cavern. The nitrogen so injected is weighed by weighing the liquid nitrogen tank during the injection process and monitoring the decrease in weight of the tank. Thus, the weight of the nitrogen injected in moving the brine-nitrogen interface from the upper level of a particular cavern section to the lower level of the section is known. By utilizing this weight and the temperature and pressure of the nitrogen, the volume that the injected nitrogen occupies in the section, and thus the volume of the cavern section, can be determined. This method involves a complex, time-consuming set of calculations along with expensive and complex equipment for weighing the liquid nitrogen tank. In addition, inaccuracies are introduced into the final results due to the necessary assumption of cavern temperature.
According to another prior method, called sonar calipering, a sonar tool is lowered into the cavern to acoustically determine the dimensions of the cavern. Although this method gives reasonably accurate results, it requires that the tubing extending into the cavern be withdrawn before testing can be done. Withdrawal of this tubing (and subsequent replacement) is an expensive operation, typically costing between $20,000 and $40,000.